The present invention relates to downhole magnetic measurement devices and methods, which are capable of being used during drilling. Downhole magnetic measurement devices may be useful for determining downhole tool orientation, well avoidance, well intercept, and verification of instrument integrity.
Accurately and precisely determining the position and orientation of a drilling assembly during drilling operations is desirable, particularly when drilling deviated wells. Traditionally, a combination of sensors are used to measure downhole trajectory and subterranean conditions. Data collected in this fashion is usually transmitted to the surface via MWD-telemetry known in the art so as to communicate this trajectory information to the surface. Many factors may combine to unpredictably influence the trajectory of a drilled borehole. It is important to accurately determine the borehole trajectory to determine the position of the borehole at any given point of interest and to guide the borehole to its geological objective. Additionally, while drilling, it is often desired to avoid collisions with other undesired objects, geological features, wells, or zones. In other cases, it is desired to intercept other desirable objects, geological features, wells, or zones. Therefore, being able to predict the presence of such features is desirable from a collision avoidance or intercept standpoint.
In some instances, surveying of a borehole using conventional methods involves the periodic measurement of the Earth's magnetic and gravitational fields to determine the azimuth and inclination of the borehole at the bottom hole assembly. Historically, this determination has been made while the bottom hole assembly is stationary as these measurements are highly influenced by rotation of the bottom hole assembly. These “static” measurements are generally performed at discrete survey “stations” along the borehole when drilling operations are suspended such as when making up additional joints or stands of drillpipe into the drillstring. Consequently, the along-hole depth or borehole distance between discrete survey stations is generally from 30 to 60 to 90 feet or more, corresponding to the length of joints or stands of drillpipe added at the surface. While there were several reasons for taking measurement-while-drilling (MWD) measurements only in the absence of drillstring rotation, a principal reason for doing so is that the sensor arrays commonly used for measurement of the drillstring's azimuth and inclination (e.g., triaxial accelerometer and magnetometer sensor arrays) yield the most reliable sensor outputs only when the drill string is stationary.
Thus, certain conventional approaches for borehole surveying take certain borehole parameter readings or surveys only when the drill string is not rotating. One undesirable consequence of this conventional approach of taking magnetometer sensor readings only in the absence of drill string rotation is that no magnetometer sensor readings are available during drilling, which can often last up to 30 to 90 feet or more. Accordingly, during the period of drilling a section of pipe, the drilling is performed without the benefit of real-time magnetometer measurements. Alternatively, drilling may be temporarily stopped to allow magnetometer measurements, but this approach results in significant loss of time, producing significant drilling delays.
There are, however, circumstances where it is particularly desirable to be able to measure azimuth and inclination while the drill string is rotating. Examples of such circumstances include, but are not limited to, (a) wells where drilling is particularly difficult and any interruption in rotation may increase drill string sticking problems, and (b) situations where knowledge of azimuth is desired to predict the real time path of the borehole.
Attempts have been made to measure the magnetic field during drill string rotation, mainly through mathematical compensation or interpolation techniques. These conventional methods, however, suffer from a number of disadvantages, namely inaccuracies, which often compound over time, resulting in some cases, wildly inaccurate and imprecise data. Thus, conventional methods suffer from an inability to detect and account for borehole deviations that occur between survey stations and/or suffer from a variety of inaccuracies and imprecision. Accordingly, improved downhole magnetic measurement devices and methods are needed to address one or more of the disadvantages of the prior art.